1. Field of the Invention:
This invention relates to an apparatus for controlling working gas pressure in a Stirling engine. More particularly, the invention relates to a working gas pressure control apparatus adapted to open an unloading valve when the rotational speed of a Stirling engine falls to a value lower than that for engine idling, when a pressure boost valve is opened, and when the engine is operating in the steady state, in such a manner as to minimize a load imposed on a compressor. The apparatus of the present invention may be effectively used for preventing engine stalling when the engine is installed in vehicles such as automobiles or forklifts.
2. Description of the Prior Art:
The output of a Stirling engine, which is an engine of the external combustion type, is determined by the pressure in a working space in which a working gas is sealed. For example, when it is desired to raise the output of a Stirling engine, the pressure of the working gas in the working space is raised. A typical prior-art output control device for a Stirling engine, such as described in the specification of Japanese patent publication No. 46-23534, is shown in FIG. 1. Each working space 1 of the engine is connected via a check value 2 to a compressor 3 by way of a maximum cycle pressure line 4 and a separate line 15. These lines 4 and 15 are connected to each other by a pressure reducing valve 5. Each working space 1 is also connected via another check valve 6 to the compressor 3 by way of a minimum cycle pressure line 7 and a separate line 16. These lines 7 and 16 are connected to each other by a pressure boost valve 8. Numeral 9 denotes a high pressure tank.
The downstream side of the pressure boost valve 8 is connected to a feedback piston cylinder 10, the piston 10a of which is connected via a piston rod 10b to one end of an accelerator lever 11, which serves as an actuating lever. A valve stem 12 of the pressure boost valve 8 and a valve stem 13 of the pressure reducing valve 5 are disposed in facing relation with respect to the sides of the accelerator lever 11. The piston 10a is moved as a function of the pressure in the feedback piston cylinder 10 in such a manner as to shift the position of a fulcrum 14 of the accelerator lever 11.
When it is desired to raise the output of the Stirling engine, the accelerator lever 11 is pushed leftwards to open the pressure boost valve 8 in order to supply the pressurized working gas to the working space 1 from the compressor 3 or tank 9. Conversely, when it is desired to lower the engine output, the accelerator lever 11 is pushed rightwards to open the pressure reducing valve in order to vent the pressure in the working space 1 toward the compressor 3 and lower the pressure within the working space 1. When the fluctuation in the engine output is within a predetermined range, that is, when the engine enters a stabilized steady-state phase of operation, the movable fulcrum 14 is shifted to close the pressure boost valve 8 and the pressure reducing valve 5.
During such steady-state operation, the compressor 3 operates to draw the working gas from line 15 and to discharge the pressurized working gas to line 16. This produces a large pressure differential between the lines 15 and 16, namely a large pressure change, which results in a greater work load upon the compressor 3 and a greater impediment to compressor operation. In order to overcome this inconvenience, it has been proposed in the the specification of Japanese patent publication No. 45-3124 to dispense with the pressure reducing valve 5 and to provide a bypass line 17 between lines 15 and 16 and a valve 18 in the bypass line 17. In accordance with this proposed system, the valve 18, which serves as the pressure reducing valve, is opened manually during steady-state engine operation to substantially equalize the working gas pressure between the lines 15 and 16. As a result, the working gas is returned during the steady-state operation from line 16 to compressor 3 by way of lines 17 and 15 so that the compressor 3 is substantially relieved of a compressive load. This makes it possible to eliminate the aforementioned impediments to compressor operation. However, this system is not completely satisfactory when one considers the operational impediments to which the compressor 3 may be subjected if one fails to perform the painstaking operation of manually opening the valve 18 when the engine begins operating in the steady state and manually closing the valve 18 at deceleration.
Accordingly, Japanese patent publication No. 46-23535 teaches to provide two operating levers 11 one of which is used to control the opening and closing of the bypass valve 18. However, this proposed system requires that the lever be operated manually to open and close the bypass valve 18. Moreover, a considerable manual force is required to operate the lever because of the pressure differential between the upstream and downstream sides of the bypass valve 18.
In addition to the inconvenience of requiring a large operating force for the actuating or accelerator lever, the aforementioned prior-art systems are inconvenient in that when the rotational speed of the engine falls below that for engine idling in the idling state, the engine may stall because of the load applied to the compressor.